US5051880A - Mixed mode regulation controller for a resonant power converter - Google Patents

Abstract

Frequency variation in a resonant zero voltage switching (ZVS) power converter is combined with supplemental duty cycle modulation to obtain voltage regulation and to narrow the required frequency band needed for regulation of a given output load. These two regulation processes are applied simultaneously to the power switch control in order to accommodate a wider range of regulated load. Hence in the alternative, for a given load range a smaller frequency range is needed than would be the case if frequency variation alone is used.

The regulation control circuit includes both a duty cycle modulator and a frequency modulator each being responsive to an error signal responsive to a differential between an output voltage of the converter and a reference voltage. Each individual modulator has its own transfer function; one transfer function being kHz/volt the other transfer function being % duty cycle/volt. The regulation control circuit output is a rectangular pulse type signal that contains both elements of frequency modulation and duty cycle modulation.

Description

FIELD OF THE INVENTION

This invention relates to a resonant switching mode power converter and in particular to a regulation controller for a resonant power converter.

BACKGROUND OF THE INVENTION

Resonant power converters are uniquely suited for applications requiring a power supply with high power density. High power density is achievable in a resonant power converter because the cyclic energy storage components of the power supply for a given power capability can be significantly smaller at very high frequencies.

Resonant converters operate by utilizing a series tuned reactive circuit as a reactive divider in combination with a load network. The output load voltage is inversely proportional to the switching frequency. Frequency of switching is always above a minimum threshold so that the series tuned network is an inductive impedance. If the load decreases in value and the input line voltage increases the frequency of operation is increased to increase the reactive division ratio to maintain regulation of the output voltage. As the load decreases further the frequency increases to a value at which high component losses result in a considerable loss in efficiency in the converter.

The need to restrain the range of frequency variation limits the load range that can be accepted, if operational efficiency is to be maintained. This limitation of acceptable load range is particularly aggravated when the load demands a wide range of output currents.

SUMMARY OF THE INVENTION

Frequency modulation is simultaneously combined with duty cycle modulation in a resonant power converter to narrow the required frequency band needed for regulation within a given range of output load. These two regulation processes are simultaneously combined and a combination regulation signal is applied to the power switch control. For a given load range ratio Rl max/RL min a smaller frequency variation range fmax/fmin is needed when duty cycle modulation is simultaneously combined with frequency modulation as compared with using frequency modulation alone.

In a specific embodiment the regulation control circuit includes both a duty cycle modulator and a frequency modulator each being responsive to an error signal responsive to a differential between an output voltage of the converter and a reference voltage. Each individual modulator has its own transfer function; one transfer function being kHz/volt the other transfer function being % duty cycle/volt. The regulation control circuit output, which in the illustrative embodiment is taken from the duty cycle modulator, is a rectangular pulse type signal that contains both elements of frequency modulation and duty cycle modulation.

BRIEF DESCRIPTION OF THE DRAWING

In the DRAWING:

FIG. 1 is a block schematic of a resonant power converter embodying the principles of the invention;

FIG. 2 is a circuit schematic of the power train of the resonant power converter of FIG. 1;

FIG. 3 is a circuit schematic of the regulation control circuitry of the resonant power converter of FIG. 1;

FIG. 4 is a graph illustrating operating characteristics of the regulation control circuit of FIG. 3; and

FIG. 5 is a graph illustrating transfer characteristics of the invention limiting circuit shown in FIG. 3.

DETAILED DESCRIPTION

A resonant power converter embodying the principles of the invention is shown in FIG. 1. AC line power is applied to theinputs terminals 101 and 102. Afull wave rectifier 10 rectifies the input AC voltage and asubsequent filter circuit 20 is included for preventing transmission of high frequency signals either into or out of the converter. The DC voltageoutput offilter circuit 20 is applied to apower switching circuit 30.Power switching circuit 30 is coupled to a series tunedcircuit 40 having a substantially solely reactive impedance and tuned to be inductive withinthe operating frequency range of the converter. Its primary function is to transfer engery chopped by thepower switching circuit 30 to a transformer50 at a single frequency. The output of the series tunedcircuit 40 is applied to atransformer 50 included in the converter to provide galvanic isolation between the input and output of the converter. Arectifier 60, coupled to receive the output oftransformer 50, charges anenergy storagecapacitor 70 to a regulated DC voltage. This DC voltage is coupled via afilter circuit 80 to theoutput terminal 191 of the converter to which a load to be energized is connected.

A regulation control for the converter includes anerror amplifier 90, which is connected to theoutput terminal 191, generates an error signal responsive to a deviation of the voltage atoutput terminal 191 from its desired regulated value. It generates an error voltage which is applied toaninverter limiter circuit 100. Theinverter limiter circuit 100 defines upper and lower boundaries to establish boundary operational conditions for the operation of thefrequency modulator 110 and theduty cycle modulator 120, to which the output of theinverter limiter circuit 100 is simultaneously directed.

Thefrequency modulator 110 and theduty cycle modulator 120 each have a specific transfer function operative in response to the error voltage output of theerror amplifier 90 as applied, via theinverter limiter circuit 100. Thefrequency modulator 110 responds directly to the error voltage output of theinverter limiter circuit 100 and produces an error voltage to signal frequency transformation. The subsequentduty cycle modulator 120 also responds to the error output at the inverter limiter circuit and applies an error voltage to duty cycle transformation to the frequency modulated output signal of thefrequency modulator 110.

The combined frequency modulated and duty cycle modulated control signal atthe output of theduty cycle modulator 120 is applied to amodulator outputstage 130 which provides two duty cycle and frequency modulated output pulse trains of opposite phase onleads 141 and 142 to theswitch drive circuitry 140. Theswitch drive circuitry 140 provides the drive signals onleads 144 and 145 to control the conductivity of two alternately switched power switches in thepower switching circuit 30.

A circuit schematic of the power train of the converter is shown in FIG. 2.A source of AC voltage is coupled to afull wave rectifier 10, via the inputs leads 201 and 202. Its rectified output is coupled, via afilter circuit 20, to thepower switch circuit 30 which circuit includes first and second MOSFETpower switching devices 231 and 232. These switching devices are connected in a half bridge configuration. The bridge capacitors are included in theswitch drive 140. The signal output at node235 is applied to a series tunedcircuit 40 comprising thecapacitor 241 and theinductor 242. The series tunedcircuit 40 is operated above resonance and hence appears to be slightly inductive to a degree dependenton the operating frequency value within the operating frequency range of the power converter. This series tunedcircuit 40 transmits power to theprimary winding 251 of atransformer 50 at a single frequency. It is also operative to control the discharging of the parasitic capacitances of the power MOSFET power switching devices with a minimum of power dissipation in these power switching devices. The output of thesecondary winding 252 oftransformer 50 is rectified inrectifier 60 and stored on avoltage storage capacitor 70. Power flow to theoutput terminals 281 and 282 is via theoutput RFI filter 80.

Switch drive to the MOSFETpower switching devices 231 and 232 is supplied by thedrive circuit 140 which is responsive to the high and low drive signals supplied atleads 248 and 249. The source of these high and low drive signals is the control circuit shown in FIG. 3. A signal proportional to the output voltage of the power train acrossleads 281 and282 is applied to theinput terminal 301 and applied to the error amplifier90.Error amplifier 90 includes aop amp 391 having sufficient loop gain and frequency compensation to insure that the control circuit loop is operationally stable for the expected range of line and load conditions. The loop gain and bandwidth is great enough so that the regulation controlmay respond rapidly and accurately to transient line and load changes. A voltage reference is applied to the non invertinginput 392 ofamplifier 391 and the voltage proportional to the output voltage is applied to invertinginput 393.

The error signal output of theerror amplifier 90 is applied to the invertinginput 303 of anoperational amplifier 302 included in theinverter limiter 100. Theinverter limiter 100 is operative to control an operational range at the converter and assure optimum control at start up of the power converter. Since the reference voltages typically achieve a normal value before the output voltage of the converter has attained a value within its desired operating range, the output of the error amplifier is initially at very high voltage. This high error voltage wouldresult in the regulation control initially driving the coverter at a high frequency and a very low duty cycle, at start up, thereby delaying attainment at a regular output. For start up the best combination is a lowconveyor frequency just above the resonant frequency of the series tuned circuit with a duty cycle of 50%. This combination permits the converter to achieve an operating line and load range most rapidly. Theinverter limiter circuit 100 assures this desired operational condition by inverting the output of theerror amplifier 90 and hence a low frequency high duty cycle is assured at start up of the power converter.

The effect of theinverter limiter circuit 100 is made readily apparent by referring to the diagram of FIG. 4.Horizontal line 401 represents the reference voltage applied to theerror amplifier 90.Horizontal line 402 represents this reference voltage as drooped by resistance 404 and appliedto the non invertinginput 304 ofoperational amplifier 302.Line 405 defines how a high error voltage output of the error amplifier onverticalaxis 410 equates to a modulation control voltage producing a minimum operational frequency onvertical axis 411.Line 406 equates even a lessererror voltage to the same miniumum frequency of operation. Within the rangeof error voltages defined by the intersections oflines 405 and 406 with theerror voltage axis 410 theoperational amplifier 305 functions as a feedback amplifier which boosts the voltage level applied to the non invertinginput 304 of theoperational amplifier 302 and maintains its output sufficiently high at substantially a fixed value to assure at leasta minimum converter operating frequency and a 50% duty cycle of operation. Below the error voltage at which theline 406 intersects theerror voltageaxis 410, the output ofoperational amplifier 305 bottoms out and its feedback action is terminated. Any further decrease in the error voltage is accompanied by a corresponding change in the voltage applied by theinverter limiter 100 to thefrequency modulator 110 and theduty cycle modulator 120.Connecting line 407 illustrates how a low error voltage results in effecting the maximum operating frequency of the converter.

The operation of theinverter limiter 100 may be readily appraised by reference to the graph of FIG. 5 which defines its transfer function. As shown, at high error voltage inputs the output voltage is substantially constant at avoltage level 501. As the error voltage input descends to athreshold value 502 the output voltage of thelimiter inverter 100 linearly rises with a decreasing error voltage as shown by the risingvoltage slope 503.

The output of theinverter limiter 100 is applied to thefrequency modulator 110 vialead 306, and to theduty cycle modulator 120 vialead 307. Thefrequency modulator 110 includes anoperational amplifier 311 operative as a voltage follower and having itsnon inverting input 312 connected to lead 306 to receive the output of theinverter limiter 100. Its output is connected to atransistor 308 which drives acurrent mirror circuit 313 operative as a voltage controlled current source and having itsoutput transistor 316 connected to supply a charging current to acapacitor 315. The rate at which capacitor 315 charges is hence dependent on the output voltage level of theinverter limiter circuit 100. Since thecurrent mirror provides a substantially constant charging current the voltage ofcapacitor 315 increases linearly during charging.

The voltage ofcapacitor 315 is applied to the non invertinginput 316 of theoperational amplifier 317 whose output in turn controls the conductivity of a discharge path controltransistor 318 which is connectedin order to dischargecapacitor 315. The voltage of thecapacitor 315 is compared byoperational amplifier comparator 317 with a reference voltage applied to itsinverting input 319 and upon attainment of that reference voltage the output ofamplifier 317biases transistor 318 into conduction thereby dischargingcapacitor 315. The recovery time ofamplifier 317 is sufficient to assure thatcapacitor 315 is fully discharged. As soon as the transistor is again biased non conducting, thecapacitor 315 begins charging again. The rate of charging ofcapacitor 315 established by the error voltage level determines the frequency of switching of the power converter.

The output of the frequency modulator is a sawtooth signal whose frequency is determined by the output voltage level of the inverter limiter. This sawtooth waveform is the operative clock signal controlling the converter frequency. It is applied to the inverting input of anoperational amplifier 321 of theduty cycle modulator 120.Amplifier 321 compares the sawthooth waveform with a reference voltage responsive to the output voltage of the inverter limiter circuit which is applied to itsnon inverting input 322.

The output ofvoltage limiter 100 is applied to the invertinginput 324 ofoperational amplifier 323 whose output controls the level of the referencevoltage applied to the non invertinginput 322 ofoperational amplifier 321. Hence, as the reference voltage changes in response to the output of theinverter limiter circuit 100 the duty cycle of the output of theduty cycle modulator 120 changes with the duty cycle modulated signal operativeat the frequency established by thefrequency modulator 110.

The resulting combined frequency and duty cycle modulated signal at theoutput lead 329 of theduty cycle modulator 120 is applied to themodulator output stage 130. Themodulator output stage 130 contains two parallel signal processing paths designated a high side drive path 331 anda low side drive path 332. Oppositely phased drive signals designated high side and low side drive signals appear on theleads 341 and 342 respectively. These two drive signals are applied to a power switch drive circuit such as is shown by the switch drive circuit in FIG. 2 which drives the alternately switched power switching devices of the power converter.

Claims (3)

We claim:

1. A resonant power converter, comprising:

a power switching circuit interconnecting an input to an output,

control circuitry for regulating an output of the power switching circuit by controlling switching in the power switching circuit, including;

an error signal generator for establishing a desired output signal level and generating an error signal proportionate to a difference between an actual output signal level and the desired output signal level;

a frequency generator responsive to the error signal for generating a frequency signal whose frequency is responsive to the error signal;

a duty cycle generator responsive to the error signal and to the frequency signal for generating a duty cycle modulated pulse signal whose frequency is established by the frequency signal with the duty cycle pulse signal generated at the frequency established and controlled in response to the error signal;

drive circuitry for driving the power switching circuitry in response to the duty cycle modulated signal;

error boundary limit circuitry for inverting the error signal applied to the frequency generator and the duty cycle generator with defined limits, at start up of the resonant power converter and including;

an amplifier circuit responsive to the error signal generator to produce an inverted error signal, and

a feedback circuit operative for retaining the inverted error signal output of the amplifier within specified limits.

2. A resonant power converter, comprising:

an input and an output;

a power switch for coupling energy between the input and the output;

a sensing circuit for quantifying a deviation of a voltage at the output from a regulation value and generating an error signal;

drive circuitry for driving the power switch, including;

a frequency modulator generating a periodic waveform responsive to the error signal of the sensing circuit;

a duty cycle modulator responsive to the error signal of the sensing circuit to generate a signal level and to the periodic waveform of the frequency modulator;

means for combining the outputs of the frequency modulator and the duty cycle modulator to generate a power switch drive signal which is simultaneously frequency and duty cycle modulated by adjusting the signal level relative to the periodic waveform, and

control circuitry for establishing a frequency of operation and a specified duty cycle at start up of the resonant power converter including a limiter circuit operative for inverting the error signal output of the sensing circuit.

3. A resonant power converter as claimed in claim 2, and further including:

circuitry for dividing a switch drive signal into two signals of opposite polarity.

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